In recent years great strides have been made in improving automatic gas operated weapon systems of the machine gun genre. New types of ammunition feed mechanisms based on rotary sprocket systems such as disclosed in Johnson et al., U.S. Pat. Nos. 3,999,461 and 4,061,074 have made it possible to significantly decrease the number of parts in and weight of portable machine guns and simplify the replacement of worn out or broken parts, as well as the manufacture of such weapons. Such portable machine guns as disclosed in the Johnson et al., patents have various advantages, such as less recoil displacement with greater control of aim and increased accuracy. The novel design and operation of these machine guns, while manifesting significant advantages, are not completely adaptable to situations where there is a need for a weapon capable of shooting high caliber ammunition, e.g., 50 caliber or higher, from vehicles, such as jeeps, or airplanes, or from tripod mounts. In addition, since the prior weapons contain only one feed mechanism they cannot provide the capability of utilizing different types of ammunition of a specific caliber without switching ammunition belts and possibly gas operating units. Dual feed units which are known are cumbersome to work with, since they require complete disengagement and displacement of one feed mechanism to enable use of the second feed mechanism and, in addition, have relatively primitive gas operating units. The weapon system of this invention provides a dual feed mechanism, as well as other mechanisms which show advantageous properties and capabilities over known weapons, and which will be discussed hereinafter.